Electric arc device with a photoelectric starting electrode



Jan. 14, 1969 P. LANG ER 3,422,307

ELECTRIC ARC DEVICE WITH AN PHOTOELECTRIC STARTING ELECTRODE Filed Sept. 13. 1966 I D i D D United States Patent Us. or. 315-150 Int. Cl. Hb 37/02,- H05b 39/04,- H05b 41/36 Claim ABSTRACT OF THE DISCLOSURE An electric arc is struck across two electrodes in a vacuum enclosure by a laser. A photocathode in the wall of the enclosure generates an electron beam under the action of the laser which beam is directed between the electrodes to trigger the are.

The present invention is concerned with a method of striking an electric are for generating very short highvoltage pulses and a device for the practical application of said method.

It is possible by means of said method to generate in times of less than 20 nsec. electrical pulses of to 20 kv. having lengths of l to 10 nsec. and leading-edge or rise times of less than 0.5 nsec. The method is of particular interest in photography of plasma bursts which have a very short lifetime.

In order to obtain voltage pulses of such a high order, it is known to have recourse to a circuit arrangement comprising two sections of coaxial line, wherein one section is connected to a high voltage D.C. supply, the other section is connected to the load of the output circuit, and both sections lead to the two electrodes of a spark gap; the production of an electric are between these two electrodes generates a high voltage pulse in said output circuit. The response time of the system and the leading-edge slope or steepness of said high voltage pulse are governed by the method adopted for initiating the arc. In point of fact, it is not usually possible by means of known methods to initiate an electric are which will produce very short pulses. This is the case in particular with one method frequently employed for establishing an are between main electrodes in a gaseous medium, which consists in ionizing the gas by applying a voltage between an auxiliary electrode and one of the main electrodes which is located in proximity thereto. The ions and electrons which are thus produced then multiply by avalanche effect and make it possible to establish the arc. However, the instants of arc formation are ill-defined and the leading-edge pulse times are substantial.

The present invention is directed to a method and device which now make it possible to produce pulses having both a very short duration and a high voltage.

More specifically, this invention is concerned with a method of striking an electric arc with a view to generating very short high-voltage pulses whereby an arc is produced between a first electrode which is connected to a high voltage supply and a second electrode which is connected to the load circuit, said method being essentially characterized in that an electron beam is directed in the vicinity of two electrodes which are placed in a vacuum enclosure and between which is applied a voltage of slightly lower value than the inter-electrode voltage which corresponds to the arc discharge.

According to a preferred mode of application of the method, said electron beam is generated by the impact of a light beam on a photocathode, for example by the impact of all or a part of a laser beam.

3,422,307. Patented Jan. 14, 1969 The present invention also relates to a device for the practical operation of the method as applicable to a spark gap constituted by a first electrode which is connected to a high voltage supply and by a second electrode which is connected to the load circuit, said device being essentially characterized in that it comprises a laser, a vacuum enclosure containing said spark gap, one of the walls of said enclosure being constituted by a photocathode which is located in the path of the light beam of said laser and which generates an electron beam from said light beam, and means housed in said enclosure and adapted to focus said electron beam between the spark gap electrodes.

In accordance with a particular embodiment, said device additionally comprises a thin semi-reflecting plate which is interposed between the laser and the photocathode and so oriented that said photocathode receives only that portion of the light beam of the laser which is reflected by said plate.

Other properties and advantages of the present invention will become readily apparent from a study of the following description together with the illustrative embodiment of the device according to the invention which is shown in the single figure of the accompanying drawlngs.

The aforesaid device, which makes it possible to produce a very short pulse of high voltage, is applied to a spark gap constituted by an anode 12 and a cathode 13.

The anode 12 is connected by means of the central conductor 14 of a section of coaxial cable in which the outer conductor 15 is connected to ground, and also through a resistor 16, to the output of a high-tension generator 17. The cathode 13 is connected through the central conductor 18 of a second section of coaxial cable to a load circuit 19 to which it is desired to apply a pulse which has both a small length and high voltage.

The device in accordance with the invention is made up as follows:

A laser 1 which generates a light beam 2; a glass plate 3 for reflecting a fraction 4 of said light beam which passes through two lenses 5 and 6, then impinges on a photocathode 7 which constitutes one of the walls of a vacuum enclosure 8 containing the spark gap and two electrodes 9 and 10 for focussing the electrode beam 11 'which emerges from the photocathode 7.

The problem to be solved by means of the system in accordance with the invention can be stated as follows:

In the system of electrodes 12, 13 which are placed in a vacuum, an arc discharge takes place at the moment of a sharp increase in the number of electrons produced by cold emission from each site of the cathode 13. When the critical current density is attained, these electrons vaporize as they strike the metal of the anode 12, thereby initiating an electron multiplication by avalanche effect; this critical density corresponds to a given voltage V between the anode 12 and the cathode 13.

The device according to the invention which makes it possible to solve this problem operates in the following manner: the voltage between the spark gap electrodes is first of all set at a threshold V, which is slightly lower than V The electron beam which is produced by the photocathode 7 under the action of the incident light beam 4 and focussed between the electrodes 12 and 13 produces an instantaneous supply of electrons at the level of the anode 12. An instantaneous current of .a few hundred ,uA is suflicient to trigger the spark gap and to generate within the conductor 18 the desired pulse which is both very short and of high voltage.

The photocathode 7 is formed of deposits which have good spectral sensitivity in the red or infrared regions. If a photocathode of substantial thickness is employed which is capable of producing in pulses of 1 as. an

instantaneous current of 0.45 a./cm. and which has a quantum efliciency of 1 ma./ W. in respect of a light-beam wavelength of 1.06 it is necessary to have O.45 10 w./cm. in order to trigger the pulse, namely the fraction 4.5)(10 of the light energy of a 10 mw. laser. The response time of the device is in that case approximately 16 nsec.

The pulses obtained can serve in particular to release the shutter of an electronic camera for the purpose, for example, of taking photographs of plasma bursts generated by the impact on a target of the non-reflected portion 20 of the laser beam.

What I claim is:

1. A device for striking an electric arc across a spark gap for generating very short high-voltage pulses comprising a first electrode connected to a high-voltage supply, a second electrode connected to a load circuit, a laser, a vacuum enclosure containing said electrodes, a photocathode in the wall of said enclosure in the path of the light beam of said laser, said photocathode generating an electrode beam from said light beam means in said'en closure for focusing said electron beam between said electron beam from said light beam, means in said entween said laser and said photocathode and reflecting a portion of the light beam of said laser onto said photocathode.

References Cited UNITED STATES PATENTS 3,141,111 7/1964 Godlove 315--181 3,294,970 12/1966 Jenckel 313-63 X 2,732,514 1/1956 Henderson 313100 X 3,295,011 12/1966 Barbini 315'156 X ROBERT SEGAL, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.R. 313100; 315-156 

